A fuel cell is a device that generates electricity by electrochemical reaction between hydrogen included in a hydrocarbon-based fuel such as methanol, natural gas, liquefied petroleum gas, etc. and oxygen included in air. It is highly esteemed as one of clean, next-generation power-generating systems. The fuel cells are divided into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte fuel cells, alkaline fuel cells, or the like. Basically, these fuel cells operate based on the same principle.
Among them, the polymer electrolyte fuel cell [PEFC, also known as proton exchange membrane fuel cell (PEMFC)] is more efficient and provides higher current density and power density than other fuel cells. In addition, with such advantages as fast startup and fast response to load change, it is widely applicable as power source of zero-emissions vehicles, private power generation, and mobile or military applications.
Referring to FIG. 1, a polymer electrolyte fuel cell has a membrane electrode assembly (MEA) 100 in its innermost location. The membrane electrode assembly 100 consists of a polymer electrolyte membrane 10 capable of conducting protons, and electrode catalyst layers applied on both sides of the polymer electrolyte membrane 10 to allow reaction of oxygen and hydrogen, i.e. a cathode 20b and an anode 20a. At the outer portion of the membrane electrode assembly 100, i.e. at the outside of the cathode 20b and the anode 20a, are provided gas diffusion layers (GDLs) 30a, 30b. And, separators 40a, 40b equipped with flow channels to allow supply of fuel and discharge of water produced by the reaction are provided outside the gas diffusion layers 30a, 30b. 
At the anode 20a of the fuel cell, hydrogen is oxidized and, as a result, a proton and an electron are produced. The proton and the electron travel to the cathode 20b through the polymer electrolyte membrane 10 and a wire, respectively. Simultaneously, at the cathode 20b, oxygen is reduced by accepting the proton and the electron from the anode 20a to produce water. Electrical energy is generated by the electron traveling through the wire and the proton traveling through the polymer electrolyte membrane 10.
A gasket 50 is provided to prevent leakage of hydrogen and oxygen.
A fuel electrode which contacts hydrogen comprises the anode 20a and the gas diffusion layer 30a provided beside the anode 20a, and an air electrode which contacts oxygen or air comprises the cathode 20b and the gas diffusion layer 30b provided beside the cathode 20b. 
Since a polymer electrolyte fuel cell used in vehicles requires a higher voltage, the fuel cell units need to be stacked to produce appreciable voltage level. FIG. 2 schematically shows a polymer electrolyte fuel cell stack formed thereby.
FIG. 2 is a cross-sectional view schematically showing a polymer electrolyte fuel cell stack 1000 according to the conventional art.
Referring to FIG. 2, the polymer electrolyte fuel cell stack 1000 comprises one or more fuel cell unit(s) comprising the membrane electrode assembly 100, the gas diffusion layers 30a, 30b and the separators 40a, 40b and end plates 70 provided at the outermost portion of the fuel cell stack.
The stack 1000 comprises one or more of the fuel cell unit(s) stacked with the membrane electrode assembly 100 and the gas diffusion layers 30a, 30b between the separators 40a, 40b and the end plates 70 attached on either side of the outermost separators of the fuel cell stack.
A (+) or (−) terminal is formed on the end plate 70.
The fuel electrode and the air electrode of the polymer electrolyte fuel cell are fabricated from a mixture of a proton conductor such as Nafion and a catalyst such as platinum. When the polymer electrolyte fuel cell is fabricated and then operated initially, its activity may decrease.
It is because, first, the reactants cannot reach the catalyst due to the blocked path for a reactant, second, the proton conductor, such as Nafion, forming a triple phase boundary with the catalyst is not easily hydrated in the early stage of operation, third, continuous transport of protons and electrons in not ensured, and, fourth, impurities included during the fabrication of the electrodes result in decreased catalyst activity.
For these reasons, in order to ensure the best performance of the polymer electrolyte fuel cell, the procedure of activation is necessary after combining the polymer electrolyte fuel cell stack using the membrane electrode assembly and the separators.
The activation is carried out to activate the catalyst which does not participate in the electrochemical reaction and sufficiently hydrate the polymer electrolyte membrane and electrolytes included in the electrodes to increase mobility of protons.
The activation performed to ensure the best performance of the polymer electrolyte fuel cell stack after assembling of the stack may take hours or days depending on the operation condition. An inappropriate activation may not ensure best performance of the fuel cell.
Such an inappropriate activation leads to decreased production rate in mass production of the polymer electrolyte fuel cell and consumption of a large quantity of hydrogen. As a result, the unit cost of the stack increases and the performance of the stack is degraded.
In general, the activation of the polymer electrolyte fuel cell is performed by load variation such as constant-voltage/constant-current cycles, according to instructions characterized by the manufacturers of the corresponding membrane electrode assembly and stack.
Usually, the activation is performed for hours depending on the activation condition of the polymer electrolyte fuel cell stack. This becomes an obstacle to mass production of the polymer electrolyte fuel cell. The number of stacks that can be produced in unit time is restricted since the stack has to remain on the conveyor line for hours during the activation procedure.
In addition, considerable time, hydrogen, air, equipments and labor are required to activate the stacks, each comprising tens to hundreds of the fuel cell units.
Especially, if the activation of the stack is performed soon after the assembling, more time and energy may be consumed because it is difficult to provide a sufficiently humid condition and an electrical load with the stack being equipped in the fuel cell system.
In order to improve the stack productivity, a plurality of stacks may be activated at the same time using a plurality of activation apparatuses. However, this inevitably increases investment in facilities and the production cost.
Meanwhile, if leakage resulting from the physical damage of the membrane electrode assembly is detected after the assembling of the stack, the stack should be disassembled and the corresponding membrane electrode assembly has to be replaced, which is not a simple task.